The use of an intermediate transfer member in electrophotography has been known for many years. Such intermediate transfer members can be provided in the form of belts or drums, and can provide a number of advantages in electrophotographic imaging including simplified receiver element handling, single pass duplexing, reduced wear of photoconductors, and superposition of multiple images to form multicolor images. As multicolor electrophotography has developed in recent years, the toners applied and fixed for multicolor images have been reduced in size in order to improve image resolution. However, this has increased the difficulty in transferring toner efficiently and accurately.
In electrophotographic formation of multicolor images, a plurality of different color toners is used. These different color toners necessitate the formation of separate electrostatic latent images on the primary imaging member and the development of respective electrostatic latent images with the proper colored toner. For example, in full-process color methods, latent image separations and toner colors corresponding to the subtractive primary colors, cyan, magenta, yellow, and black, are used. These separations must ultimately be transferred to a receiver member in register in order to form the multi-color image reproduction.
In many multicolor electrostatographic or electrophotographic reproduction apparatus, transferring separate colors to a receiver member is accomplished by wrapping the receiver member around an electrically biasable drum. The electrostatic latent images, which have been formed on separate areas of the photoreceptor that correspond to the periodicity of the drum, are each rendered into visible images using the separately colored toner particles. These images are then transferred, in register, to the receiver member. This process, however, has a complicated receiver member path, as the receiver member must be picked up and held by the transfer drum and then released back to the transport mechanism at the appropriate time. This process can be simplified by first transferring all the separate images, in register, to an intermediate transfer member and then transferring the entire composite image to the receiver member. In either of these two modes of operation, the output speed of the electrostatographic reproduction apparatus is reduced due to the number of sequential transfers that need to be done.
In another example of color electrostatographic reproduction apparatus, it is desirable to separate the color separation image formation process into separate and substantially identical modules. This allows each colored image to be printed in parallel, thereby increasing the speed of the reproduction apparatus. In this process, the receiver member is transported from module to module and, while it can be picked up and wrapped around a transfer roller, there generally is no need to do so. It is also desirable to firstly transfer each image to an intermediate transfer member, such as a compliant transfer intermediate member as described in U.S. Pat. No. 5,084,735 (Rimai et al.). In order to reduce the time needed to produce a printed image, it is further desirable, however, that each color is produced in a separate module comprising a primary imaging member, development station, and transfer apparatus.
In all of these processes, it is necessary to transport the receiver member through the electrostatographic reproduction apparatus. One mode of transport uses a transport web such as a seamless transport web to which a receiver member can be attached electrostatically or by any other well known mechanism. When such a transport web is employed, in order to facilitate registration of individual developed images on a receiver member, it is desirable to drive the image forming modules using friction, especially in the case where separate modules are used for the formation, development, and transfer of individual color separation images. This requires that the web have a sufficiently high coefficient of friction during operation as described in U.S. Pat. No. 7,252,873 (Ferrar et al.). It also requires that the intermediate transfer member have a high coefficient of friction against the photoreceptor. Although many compositions can have sufficiently high frictional coefficients initially, the presence of fuser release agents on the receiver member transport web can reduce the friction with increased usage and result in slippage in a frictionally driven electrostatographic reproduction apparatus. This can result in image defects such as mis-registration and general overall unreliability of the reproduction apparatus.
In other reproductive methods, it is necessary that a high degree of slip exists between the different components of the printer. This allows for differences in speed between the photoreceptor, intermediate transfer member, and the receiver or transport member. A low coefficient of friction is desirable for these situations where the members pass each other at different rates but the color registration is not deleteriously affected by the drag of one surface on another.
An intermediate transfer member generally includes a substrate on which is formed a relatively thick, resilient blanket or compliant layer, and a thinner outermost surface layer on which toner is held. The compliant layer is generally composed of an elastomeric polymeric material such as a polyurethane that facilitates contact of toner particles with the member because of its desired deformation properties. The compliant layer can be electrically modified to enhance the electrostatic attraction of the toner particles. Since polyurethane compliant materials do not readily release toner particles, the relatively thin outermost surface layer (or “release” layer) is necessary for the member to be effective. It would be further desirable that the electrical conductivity of the intermediate transfer member be relatively independent of humidity conditions.
Several properties of the intermediate transfer member surface are especially important. Firstly, the surface energy should be sufficiently low to facilitate release of the fine toner particles. In addition, the intermediate transfer member surface should have good wear properties against the highly abrasive conditions of the transfer process. During the transfer, pressure is exerted on the toner particles at the first nip formed by a photoconductor and the intermediate transfer member. Even higher pressure is typically exerted at the second nip, where a receiver element, most often a paper sheet, is brought into contact with the toner particles on the intermediate transfer member surface. Residual toner particles are removed at a cleaning station that may include a blade, fur brush, or magnetic brush.
The outermost surface layer of the intermediate transfer member should also have sufficient flexibility to prevent cracking during the toner transfer process. The hardness of the substrate and compliant layer on which the outermost surface layer is disposed can vary over a considerable range, so it is necessary to adjust the flexibility of the outermost surface layer appropriately. This outermost surface layer is sufficiently thin or static dissipative to prevent its acting as an insulator against development of the field necessary for electrostatic attraction of the toner particles. It should also not work against the compliant layer properties.
In summary, it is important to control the surface energy, wear, electrical resistivity, humidity dependence, and flexibility properties of the intermediate transfer member. These properties can be evaluated by, respectively, contact angle measurements, abrasion test measurements, electrical resistivity, and storage modulus determination.
There are dozens of publications that describe various intermediate transfer member constructions and composition including, but not limited to, U.S. Pat. No. 5,084,735 (Rimai et al.), U.S. Pat. No. 5,337,129 (Badesha), U.S. Pat. No. 5,480,938 (Badesha et al.), U.S. Pat. No. 5,525,446 (Sypula et al.), U.S. Pat. No. 5,689,787 (Tombs et al.), U.S. Pat. No. 5,714,288 (Vreeland et al.), U.S. Pat. No. 5,728,496 (Rimai et al.), U.S. Pat. No. 5,985,419 (Schlueter, Jr. et al.), U.S. Pat. No. 6,548,154 (Stanton et al.), and U.S. Pat. No. 6,694,120 (Ishii), EP 0 747 785 (Kusaba et al.), and U.S. Patent Application Publication 2004/0247347 (Kuramoto et al.). In addition, U.S. Pat. No. 5,968,656 (Ezenyilimba et al.) describes intermediate transfer members having an outermost surface layer that includes a ceramer comprising a polyurethane silicate hybrid organic-inorganic network.
While the noted ceramer-containing intermediate transfer member has been used commercially and successfully for years, there is a need for improved intermediate transfer members.